The ability to quantify gaseous species, such as O.sub.2 and NH.sub.3, and analytes in solution, such as pH, PO.sub.2, PCO.sub.2, glucose, cholesterol, antigens, haptens, amino acids, and organic molecules, is important in industry, biomedicine, and the analytical sciences.
Traditionally, molecular oxygen, O.sub.2, has been sensed using a device known as the Clark electrode. Although this electrode works, it has limitations including consumption of the O.sub.2, relatively long response times, and the tendency of the electrode to become poisoned by contaminants, such as proteins and organics. As a result, other solutions, which rely upon optical sensing schemes for quantifying O.sub.2, have been developed..sup.1-21
Most optical sensing schemes are based on the quenching of a luminescent species by a gas, such as molecular oxygen..sup.1-11,22-24 In this approach, the O.sub.2 dependence, or the dependence of any other quencher like Cl-, Br-, J-, Cu.sup.2+, Ni.sup.2+, Cr.sup.2+, Fe.sup.2+, Fe.sup.3+, or acrylamide, on the emission intensity is described by the Stern-Volmer expression: .sup.25,26 ##EQU1##
where I.sub.0 is the intensity in the absence of O.sub.2, I is the intensity in the presence of O.sub.2 at concentration [O.sub.2 ], K.sub.SV is the Stern-Volmer quenching constant, k.sub.q is the bimolecular quenching constant, and .tau..sub.0 is the excited-state luminescence lifetime of the emissive species in the absence of O.sub.2. Accordingly, by monitoring the luminescence intensity, the amount of O.sub.2 present in a given sample can be determined.
Early optical sensing schemes used O.sub.2 sensors which were based on the fluorescence from polycyclic aromatic hydrocarbons (PAHs) with long excited-state lifetimes, such as pyrene, benzo[a]pyrene, pyrenebutyric acid, and decacyclene..sup.1-5,11,12 Since these fluorophores have reasonably long excited-state lifetimes (to 400 ns), they are susceptible to O.sub.2 quenching. Unfortunately, they also exhibit absorbance maxima in the ultraviolet or blue spectral region. As a result, the light sources in these optical sensing schemes consume significant electrical power and/or are expensive. Additionally, the detectors needed for these optical sensing schemes (e.g., photomultiplier tubes) are costly and require high voltage power supplies.
Other luminescent species that are susceptible to O.sub.2 quenching include platinum and palladium porphyrin complexes.sup.6-7, and ruthenium poly(pyridyl) complexes 8-10,14-15,17-21 Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II), which is commonly referred to as [Ru(dpp).sub.3 ].sup.2+, is particularly attractive for O.sub.2 sensing because it exhibits a high luminescent quantum yield, long excited-state lifetime, large Stokes shift, and strong absorption in the blue-green spectral region..sup.22-24, 27 These luminescent species have shown promise as luminescence quantum counters, as singlet oxygen generators.sup.25,26 for synthetic applications, and as sensors and molecular probes..sup.29
However, simple, small, and inexpensive optical sensing systems with these luminescent species have not yet been developed. The principal difficulties associated with constructing these sensing systems are with the immobilization of the O.sub.2 responsive species and the relatively high cost of the excitation and detection system.
One approach to overcome these difficulties involves an optical O.sub.2 sensor that uses a light emitting diode and a silica optical fiber with a sol-gel-derived film deposited on one surface..sup.17-21 The use of the sol-gel-derived film to entrap species provides a number of advantages including: (1) ambient processing conditions; (2) tunable film porosity; (3) good thermal stability; (4) optical transparency; and (5) simple dopant entrapment procedures.30-32 However, the use of the optical fiber adds to complexity and cost of the system and requires careful, precise, and costly manufacture to properly couple the light from the light emitting diode into the fiber and optically filter the fluorescence.
An alternative approach was recently described in U.S. Pat. No. 5,517,313 to Colvin, Jr., which is herein incorporated by reference. In this approach [(Ru(dpp).sub.3 ].sup.2+ is immobilized within a silicone:naptha membrane (1:2, vol:vol), and a light emitting diode is embedded directly into the membrane. In this configuration, the housing for the light emitting diode acts essentially as a waveguide to couple the light into the film. This configuration is optically simpler than the aforementioned optical fiber design,.sup.17-21 but still requires complicated flow cell and waveguide construction techniques for proper operation.